This invention relates to a method of transferring semiconductor devices or circuits fabricated on top of semiconductor or insulating layers as continuous sheets from one substrate to another. Semiconductor devices can be fabricated on top of the sheet layer being transferred and such semiconductor devices will, therefore, also be transferred to a different substrate.
This method of transfer results in maintaining the original registration between devices and circuits, which registration existed before the transfer because the underlying support layer is transferred as a sheet.
Description of the Related Art
A common, well established method of transferring semiconductor devices from their original substrate to a new substrate is known as the Epitaxial Liftoff Process (ELO) in which the devices are fabricated on top of a sacrificial release layer. The ELO method is disclosed in U.S. Pat. Nos. 5,641,381 to Bailey, et. al. and 4,846,931 to Gmitter, et. al.
According to the ELO method, by attaching the top of the semiconductor devices to an adhesive or a wax layer, the release layer is chemically etched thus releasing the devices from their original substrate. The released devices are then attached to the new substrate using another adhesive.
Both patents mentioned above teach a technique of applying a layer of a polymer or a wax on top of a device and curing this layer at specific temperature. As a result compressive stress develops in the device layer and tensile stressxe2x80x94in the polymer or wax layer leading to the curling up of the film as the sacrificial layer is etched away thus releasing both the device layer and the polymer/wax layer together from the substrate. The Gmitter patent is the original ELO patent and teaches this technique in general while the Bailey patent develops it further and describes specific means of removing InP layers by using a ternary InGaAs sacrificial layers.
The ELO method has serious disadvantages. It leads to a loss of the original interface between the bottom of the removed devices and the release layer which can negatively effect their ultimate electrical characteristics. This disadvantage is particularly acute if the device layer is very thin.
Other disadvantages are the difficulty in positioning the removed devices on specific locations on the new substrate to maintain registration between the devices and the inability in transferring prefabricated metallization between the devices, such as in active matrix arrays. In addition, the device layers released according to the ELO method are only supported by a thin and flexible polymer/wax layer often resulting in cracks and fractures in the thin device layer before bonding thus greatly reducing the fabrication yield. Finally, if the devices and/or circuits on the device layer are to be electrically isolated by forming islands in the device layer prior to transfer, the registration between these devices and/or circuits will be lost if the transfer is according to the ELO technique.
The inventor is unaware of any method which overcomes the above mentioned problems associated with the ELO method. Yet there is a need for a transfer method which will:
(a) maintain the original electrical interfaces between the device layer and the layer on top of which the device layer is fabricated;
(b) maintain the original registration between the individual devices;
(c) need no special bonding layer between the bottom of each device and the new substrate; and
(d) maintain electrical interconnections between individual devices prefabricated prior to transfer.
The method proposed in this inventions accomplishes all these tasks. As a result, the devices and circuits will operate under conditions identical to those prevalent at the pre-transfer stage without any need for further processing (other than deposition of light modulating or emitting layers in case of displays).
The present invention is directed to a multi-step method of transferring prefabricated semiconductor devices and circuits from an original substrate to a new substrate. The gist of this method is that semiconducting or insulating layers together with semiconducting devices and circuits fabricated on top of those layers are transferred from their seed substrate to another substrate as sheets. This transfer is accomplished without removing the original underlying layer on which the devices and circuits were fabricated.
The method involves the following principal steps. The devices or circuits, including potentially metallization, are fabricated on a thin active layer and an optional protective layer of a photoresist is applied over them. Either the exposed surface of the photoresist (if used) or the thin active layer with components thereon is then coated with a thin layer of a wax, which can be applied by a variety of methods. A specially prepared perforated metallic, semiconductor or plastic structure is also coated with the same wax followed by attaching this special structure to the waxed outer surface of the photoresist in such a way that the two wax layers are fused together. An important step in this process of attachment is heating the two waxed surfaces facing each other so that wax softens up thus facilitating the fusion of the two wax layers.
After this attachment of the special perforated structure, the initial substrate upon which the devices or circuits were fabricated (the seed substrate) is etched off, either by dry etching or by wet etching, and the seed substrate is removed. The final step involves the attachment of the exposed surface of the sheet semiconducting or insulating layer, on top of which the devices or circuits are fabricated, to a new permanent substrate using an appropriate adhesive, for example, an epoxy resin. The wax is then dissolved in an appropriate solvent. The perforations in the special perforated structure play an important role greatly facilitating the removal of wax.